I personally believe that evil spirits can mimic other people. I had a very good friend who has seen "things" claiming to be his sister and his best friend, both deceased, and he told me, "Kim, I know one thing. It looked like them... but it wasn't them." And no, he's not an occultic devil worshiper but rather, a Bible-believing Christian who has, even from childhood, been sensitive to supernatural happenings.
Now, I could be wrong in my thinking--it's a personal belief and not proven fact. But I find it interesting that the Bible mentions a battle between the devil and the angel Michael over Moses' body because Satan wanted to use Moses' body to deceive people.
I've also found it intriguing that when Jesus was resurrected and appeared to His disciples, they immediately thought He was a ghost. The Bible says that Jesus even ate a piece of fish in front of them, in part, to prove to them that it was really Him and that He had been restored as a true human being.
What fascinates me most is that Jesus, the Creator of the Universe, did not tell them, "You silly disciples, there are no such things as ghosts!!" But instead, He told them, "A ghost does not have flesh and blood as I have." I just always thought that if ghosts absolutely, positively, no-way-in-the-entire-world do not exist... Why didn't Jesus just say so? He never failed to correct them in false beliefs before--in fact, His answer was correcting their false belief that He was a ghost to begin with--so I've always figured if no such thing exists, Jesus would be the first to tell them. But that's not what He did.
I'm not saying ghosts DO exist... but I'm not entirely convinced they can be completely ruled out either, and may have a different definition than what we humans believe them to be. Which may be another reason why God has always strictly forbidden contact with the spirit world other than Himself.